1. Field of the Invention
This invention pertains to displays, especially liquid crystal displays (LCDs) with vertically polarized output and, more particularly, to an assembly for restricting the field of view of such displays so that the image appearing on the display is visible primarily to a user sitting directly in front of the screen and is not observable to other individuals viewing the screen at an oblique angle. This objective of viewing privacy is achieved by substantially eliminating light emanating from the screen other than in a substantially perpendicular manner.
2. Description of Related Art
Liquid crystals are useful for electronic displays because polarized light traveling through the liquid crystal layer is affected by the layer's birefringence, which can be changed by the application of a voltage across the layer. As a result, the transmission or reflection of light can be controlled with much less power than is required for the luminescent materials used in other types of displays. This contributes to the longer life, lower weight and low power consumption of the LCD displays.
In LCD computer monitor and television displays a matrix of pixels is arranged across the display. These pixels are activated by an X-Y sequential addressing scheme between two sets of perpendicular conductors. Where the displays incorporate nematic liquid crystals, arrays of thin film transistors can be used to control the driving voltages at the individual pixels.
In many applications, it is desirable to broaden the angle under which the display is viewable without distortion of loss of contrast. For example, in avionics it is important that the display be clear and undistorted to several individuals viewing the screen from various angles. In many instances, it is also desirable for a computer display to be visible to observers other than the user and for video screens to present an undistorted image to an audience sitting other than directly in front of the screen. Assemblies which will allow a greater undistorted, high intensity-viewing angle are shown in numerous prior art references, such as U.S. Pat. No. 5,612,801.
However, numerous applications are arising where it is desirable to significantly narrow the usable viewing angle to provide viewing privacy. The intention is to allow access to the image on the screen only to the party sitting directly in front of the screen and to prevent a person sitting next to the user or standing over the user from seeing the image on the screen. For example, it is now becoming commonplace for computer users to use portable computers on airplane flights to work on proprietary documents. For security reasons, it is most desirable to prevent a passenger sitting in the next seat or passing down the aircraft aisle from observing the information on the screen. Secondly, newer cabin designs in aircraft are replacing larger, centrally located viewing screens, usually hanging over the aisle, with smaller individual screens which drop from the overhead compartment, or individual screens mounted in each passenger's seat back or tray table so that each passenger can select his own entertainment. Often, because each passenger may be receiving and/or paying individually for his or her chosen entertainment display, such as a movie or access to various web sites or, for security reasons, the receipt of proprietary e-mail messages, it is desirable to prevent surrounding parties from observing the contents of the display.
Most devices developed to enhance usage of LCD displays are directed to making the image thereon available to a broader audience. Devices which can be added to the LCD device to narrow the field of view and limit the scope of the viewing audience in a simple, light weight and unobtrusive manner without loss of brightness of the image are not commonly available. Currently, to reduce the angle of vision, micro-louvers provided by 3M are used. However, they result in a reduction of image brightness that must be compensated for by increasing the power applied to the backlight and/or use of various brightness enhancing films. That deficiency is addressed by the present invention which is a privacy screen for a display comprising a rotating film, a birefringent film and a polarizing film, and entails use of the privacy screen on a display having a state of polarization in order to narrow the field of view of the display, thereby achieving privacy. The privacy screen is assembled in a manner such that the horizontal components of light rays emitted from the display screen in other than a substantially orthogonal manner are blocked by the privacy screen and not transmitted to the observer. As a result, the opportunity to view the image on the screen from other than approximately 90° is significantly reduced or eliminated. The intensity of the transmitted light may be enhanced by the addition a brightness enhancement film.
U.S. Pat. No. 6,239,853 discloses a LCD privacy screen comprising staggered waveplates that contain alternating birefringent and isotropic film portions. While the privacy screen disclosed in this patent may be effective, it has drawbacks of having a complex design of alternating birefringent and isotropic regions (as stripes), which are difficult, time-consuming and relatively expensive to fabricate. More importantly, the Rockwell patent is based on full-wave and half-wave considerations only, without consideration to changes in angle of incidence upon retardation effects. The present invention fulfills a need for an efficient and cost-effective privacy screen for the display industry.